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Running with optimizer_switch='extended_keys=on' may cause ORDER BY .. LIMIT optimizer to skip efficient query plans.
Cnsider the following query:
MariaDB [mw]> explain SELECT /* WikiExporter::dumpFrom */ * FROM `page` INNER JOIN `revision` IGNORE INDEX (PRIMARY) ON ((page_id=rev_page)) INNER JOIN `text` ON ((rev_text_id=old_id)) WHERE page_namespace=4 AND page_title='Sandbox' ORDER BY rev_timestamp ASC LIMIT 10;
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+------+-------------+----------+--------+--------------------+----------------+---------+-------------------------+------+-------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+------+-------------+----------+--------+--------------------+----------------+---------+-------------------------+------+-------------+
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| 1 | SIMPLE | page | const | PRIMARY,name_title | name_title | 261 | const,const | 1 | |
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| 1 | SIMPLE | revision | ref | page_timestamp | page_timestamp | 4 | const | 10 | Using where |
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| 1 | SIMPLE | text | eq_ref | PRIMARY | PRIMARY | 4 | mw.revision.rev_text_id | 1 | |
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+------+-------------+----------+--------+--------------------+----------------+---------+-------------------------+------+-------------+
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Here, index page_timestamp is defined as
KEY `page_timestamp` (`rev_page`,`rev_timestamp`)
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The ref_page part is constant due to use of ref(const) access. Hence, the optimizer is able to conclude that ordering required by "ORDER BY rev_timestamp" is produced by the used index.
If one sets extended_keys=on, the query plan remains the same, except that filesort is used:
MariaDB [mw]> explain SELECT /* WikiExporter::dumpFrom */ * FROM `page` INNER JOIN `revision` IGNORE INDEX (PRIMARY) ON ((page_id=rev_page)) INNER JOIN `text` ON ((rev_text_id=old_id)) WHERE page_namespace=4 AND page_title='Sandbox' ORDER BY rev_timestamp ASC LIMIT 10;
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+------+-------------+----------+--------+--------------------+----------------+---------+-------------------------+------+----------------------------------------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+------+-------------+----------+--------+--------------------+----------------+---------+-------------------------+------+----------------------------------------------+
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| 1 | SIMPLE | page | const | PRIMARY,name_title | name_title | 261 | const,const | 1 | Using where; Using temporary; Using filesort |
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| 1 | SIMPLE | revision | ref | page_timestamp | page_timestamp | 4 | mw.page.page_id | 1 | |
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| 1 | SIMPLE | text | eq_ref | PRIMARY | PRIMARY | 4 | mw.revision.rev_text_id | 1 | |
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+------+-------------+----------+--------+--------------------+----------------+---------+-------------------------+------+----------------------------------------------+
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3 rows in set (0.00 sec)
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Use of filesort may cause performance degradation. (the testcase for this bug is too small to show it, though).
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mdev-4340.sql